Audiometry is used to examine and measure functional parameters of the auditory organ and to delineate sound conduction and sound perception disorders. The variety of procedures used covers a wide spectrum from simple tuning fork tests to complex subjective and objective sound and speech audiometric procedures. Objective procedures also include electrical brainstem audiometry for objective measurement of sound sensations.
What is audiometry?
Audiometry is primarily used to detect and measure hearing disorders. Audiometry is primarily used to detect and measure hearing disorders. Since hearing disorders can have multiple causes, it is not sufficient just to determine and measure the hearing loss in simple hearing parameters such as frequency response and sound pressure, but the causes must be found out if possible in the sense of a goal-oriented therapy. Hearing loss can be caused either by problems with the external auditory canal or eardrum, or by sound conduction problems in the middle ear, or by sound perception disorders caused by weaknesses in the conversion of mechanical sound waves into electrical impulses in the cochlea. The same symptoms of a sound perception disorder can also be caused by lesions or diseases of the auditory nerve (vestibulocochlear nerve) or by problems in the further processing of the nerve impulses in the central nervous system (CNS). Therefore, there are a number of procedures and technical aids that can be used to narrow down hearing problems to sound conduction or hearing sensitivity problems. In the case of a diagnosed sensorineural hearing loss, so-called recruit measurements can be used to determine whether the problems lie in the inner ear, the auditory nerve or the processing centers in the CNS. Recruitment audiometry measures the responses of the sensory cells in the cochlea to loud and soft sounds. Soft sounds are normally amplified by self-emission and loud sounds are attenuated to protect hearing.
Function, effect, and goals
Audiometric procedures are used primarily when impaired hearing is suspected. In special cases, an audiogram is also used to provide evidence of minimum hearing, such as for pilots during their medical fitness testing. Relatively simple procedures are tuning fork tests, each named after its inventor, such as the Weber, Rinne or Bing test. Most tuning fork tests are based on subjective comparison between air and bone conduction of sound. In the tests, the tuning fork is either placed with the base on the skull or on the bony process behind the auricle, or alternately the vibrating fork tip is held in front of the auricle. Depending on the subjective hearing sensation, differences in hearing between the left and right ear can be detected and whether there is a sound conduction problem with restricted function of the ossicles in the middle ear. In principle, this is the case if the tuning fork is perceived better via bone sound than via airborne sound. Another subjective form of audiometry that is frequently used is sound audiometry, in which the sound pressure of the individual hearing threshold is recorded as a function of frequency in a diagram for the left and right ear. The hearing thresholds for airborne sound and for bone sound are measured. If the curves for bone sound show lower values (sound pressures), i.e. better hearing, there is a sound conduction problem in the middle ear. In addition to hearing distance tests (whispering speech) and investigations of the discomfort threshold, noise audiometry according to Langenbeck offers possibilities for localizing problems with a sound perception disorder. The procedure is comparable to sound audiometry, but the pure tones used to determine the hearing threshold are underlaid by noise of varying intensity. A relatively simple objective measurement method is tympanometry, which measures the elasticity and reactivity of the eardrum. Small pressure fluctuations are generated in the external auditory canal and the response of the eardrum is measured and conclusions are drawn about acoustic resistance. The measurement method requires an intact eardrum. In most cases, examination of the stapedius reflex is also included. The stapedius reflex is triggered by a loud banging noise to protect hearing.When the reflex is activated by a loud bang, a tiny muscle on the stapes contracts and tilts the stapes plate so that the sound is only processed further in reduced amplitude (muffled). Measurements of otoacoustic emissions and brainstem audiometry are especially useful for speech development disorders and for patients after strokes that have affected hearing. Otoacoustic emissions occur in the sensory cells of the cochlea in response to soft sounds, which are virtually amplified, and to very loud sounds, which are attenuated when translated into electrical nerve signals.
Risks, side effects, and dangers
Audiometric testing is always done noninvasively, with one exception. Neither are drugs or other chemical substances involved. In this respect, audiometric examinations can be classified as free of side effects and as hazard-free. Theoretically, there is a negligible risk of injury if the tuning fork is handled improperly during the tuning fork test. An equally negligible technical risk exists with the audiometers if the sonication via headphones were to suddenly reach a level that would damage the hearing. The greatest risk in the provocation and measurement of otoacoustic emissions and in the measurement of brainstem activity is possible misdiagnosis, which can occur especially in the screening of newborns. A misdiagnosis – if not unmasked as such by further investigation – can unnecessarily stress the affected parents and possibly initiate unnecessary therapy in the infant or toddler. The procedure that is the only one that can be described as invasive is electrocochleography, which measures the currents generated by sensory cells in the cochlea just milliseconds after receiving a sound as amplification. The procedure is particularly accurate when the electrodes are placed directly in the inner ear in the form of electrode needles through the eardrum rather than externally applied, so in this case it is invasive.
